This invention relates generally to gas lasers and laser cavity configurations therefor, and more particularly to a new and improved laser cavity configuration especially applicable to repetitively pulsed chemical or gaseous flow electric discharge lasers.
The present invention provides a novel laser cavity configuration which may be applicable both to chemical lasers, such as the repetitively pulsed HF/DF type, or to gaseous electric discharge type lasers, such as that utilizing carbon dioxide.
Conventional repetitively pulsed high energy lasers employing gaseous flow transverse of the optical axis have associated with their operation transverse acoustic disturbances, such as pressure or shock waves, as an undesirable result of the violent reactions occurring in the laser cavity which produce the desired high energy laser output. These disturbances are reflected off the cavity walls back into the optical cavity causing density gradients in the lasing medium which seriously reduce the optical quality of the laser beam. One known method of reducing the effects of these transverse acoustic disturbances is to employ acoustic attenuators that absorb and dampen the shock waves. However, this approach increases the size, weight, and complexity of the laser. Axial flow of the reactant gases through the laser cavity substantially reduces the effects of these transverse disturbances.
In conventional pulsed gaseous lasers a shock wave is formed in the mixing section of the nozzle through which the reactant gases are injected into the cavity. These lasers are fed by alternating slugs or pulses of gaseous laser reactants and inert gases. The inert gases assist in controlling the reaction in the cavity, i.e., achieving flameout, when a reaction is initiated by flash lamps or other suitable pump devices. The shock waves generate entropy in the mixing section of the injection nozzle. This entropy becomes a signature for each particle which travels through the cavity as a series of spaced disturbances whose spacing correlates to the timing of the pressure history of the laser cavity. These spaced disturbances, called residual entropy disturbances, reduce the beam quality in transverse lasers. However, the effects of these disturbances are minimized in axial flow lasers because the disturbances are lined up perpendicular to the optical axis.
The novel laser device of this invention is configured to utilize axial flow of the gaseous laser medium within the lasing cavity of the device to substantially eliminate the transverse acoustical disturbances which are characteristic of conventional radial or transverse flow configurations and which deleteriously affect their operation. The configuration permits operation at relatively high pressure (two or more atmospheres) in order to reduce laser pulse length and to increase laser energy density. Further, a choked exhaust exit is included to cancel cavity disturbances and acoustic wave reflections into the lasing cavity. The choked exhaust prevents shock waves generated downstream of the laser cavity from being reflected back into the optical cavity, because the exiting gases are traveling at about the speed of sound, and the sonic flow effectively blocks reflections from re-entering the cavity. The choked exit reduces cavity disturbances by allowing the disturbances to leave the optical cavity via the exit opening. Significant reductions in the cavity disturbances can be achieved in the device disclosed herein so long as the cross-sectional area of the exit opening is .perspectiveto.50% of the cross-sectional area of the lasing cavity.
The laser optics associated with the operation of the device of the present invention may conventionally comprise a window of suitable laser transparent material, disposed at each end of the lasing cavity along the lasing axis thereof and at an angle to the lasing axis corresponding to the Brewster's angle characteristic of the window material. The laser optics (mirrors, output coupler, and the like) which define the optical resonant cavity for laser beam generation may then be disposed externally of the structure defining the lasing cavity, allowing substantial reduction in total system bulk and weight. The laser may then operate with a minimum of optical losses associated with reflection losses by the windows, and, in addition, the laser beam may be polarized in a predetermined direction.
The foregoing characteristics of the laser device of the present invention provides significant advantages over prior art devices in that the need for an acoustic attenuator system downstream of the lasing cavity is eliminated, and cavity volume is reduced, both advantages resulting in significant reduction in total laser system volume and weight without sacrificing total laser power output or efficiency. Laser beam quality is improved by aligning entropy disturbances within the lasing cavity perpendicular to the lasing axis, and allowing them to exit the cavity through the open exhaust; thereby minimizing their effect. Total laser power output may be increased, as compared to existing devices, by reason of shorter pulse, higher energy density operational capability characteristic of the present invention. Additionally, inserting the gaseous reactants into the lasing cavity and exhausting them at about the same angle as that at which the windows are set improves aerodynamic and optical properties of the gaseous flow. The portion of the flow over the windows is advantageous in providing a section for fine scale mixing before the gaseous reactants enter the laser cavity. In addition, this section also provides a length for flame-out between laser pulses in which a noncombustible mixture is inserted. The mass flow utilization is significantly improved over transverse flow lasers by the favorable ratio of the laser cavity volume to the mixing section volume which is an inherent feature of the novel configuration of this invention.
It is, therefore, an object of the present invention to provide an improved gaseous laser device.
It is a further object of the present invention to provide an improved gaseous laser device having substantially improved efficiency and reduced lasing cavity acoustical disturbances associated with laser beam generation.
These and other objects of the present invention will become apparent as the detailed description of representative embodiments thereof proceeds.